Surface treatment method of hydrogen absorbing alloy

ABSTRACT

Steam is contacted with a hydrogen absorbing alloy in a temperature range from 200° C. to 400° C. With a contact catalytic reaction of water, a metal contained in the hydrogen absorbing alloy is changed to an oxide or a hydroxide. Hydrogen produced causes the Ni compound to be reduced and thereby the Ni metal that is catalytically active is produced. Thus, the surface of the hydrogen absorbing alloy is activated. The steam is contained in an inert gas or a reductive gas. This treatment method is suitable as an activation treatment for a hydrogen absorbing alloy used as an active material of a negative electrode of a secondary battery.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a surface treatment method of ahydrogen absorbing alloy, in particular, to an activation treatmentmethod of a hydrogen absorbing alloy used for an active material of abattery.

2. Description of the Related Art

A hydrogen absorbing alloy absorbs hydrogen at a higher density than ahydrogen-storing cylinder or a liquid hydrogen. In addition, thehydrogen absorbing alloy can reversibly repeat a cycle ofabsorbing/releasing hydrogen. With this characteristic, the hydrogenabsorbing alloy has been used for a heat engine that uses hydrogen asfuel, a chemical heat pump that uses heat generation/absorptioncorresponding to the absorption/release of hydrogen, a nickel-hydrogenbattery using electrochemical hydrogen absorption/release, and so forth.

As hydrogen absorbing alloys that have been used or that will be used,the LaNi₅ type, Ti--Fe type, and Zr alloy Laves phase type, thatabsorb/release hydrogen at normal temperature and at normal pressure orin the vicinity thereof, are known. In particular, the equilibriumpressure at room temperature of the AB₅ type hydrogen absorbing alloyrepresented by LaNi₅ and MmNi₅ (Mm: misch metal that is a mixture ofrare earth group elements such as lanthanum and cerium) or AB₂ typehydrogen absorbing alloy represented by TiZrVni type Laves phase alloysuch as ZrV₀.4 Ni₁.6, is approximately one atmosphere. Thus, thesehydrogen absorbing alloys reversibly absorb and release hydrogen atnormal temperature and at normal pressure. In addition, these hydrogenabsorbing alloys have relatively good corrosive resistance againstalkali solutions. Consequently, the hydrogen absorbing alloys can beused as an active material of a negative electrode of a secondarybattery that repeats the charging and discharging operations expressedby the following equation. ##STR1##

However, when the surface of the above-described hydrogen absorbingalloy is exposed to air, an oxide layer is easily formed on the surface.The oxide layer prevents the hydrogen absorbing alloy fromabsorbing/releasing hydrogen. In particular, when an oxide layer isformed on the surface of the hydrogen absorbing alloy used for thenegative electrode of a nickel-metal hydride battery, since an Nicatalyst layer that dissolves and activates hydrogen is not present, thehydrogen absorbing alloy does not easily absorb/release hydrogen. Thus,in the initial stage of the battery, it does not have an enoughdischarging capacity. In other words, when the oxide film is formed onthe surface of the hydrogen absorbing alloy, the initial activatingcharacteristic is low.

To solve such a problem, in Tokkaihei 5-13077 and 4-137361, powder ofhydrogen absorbing alloy is soaked in alkali solution so as to removethe oxide layer on the surface of the hydrogen absorbing alloy. Inaddition, mish metal, Co, Al, and Mn are dissolved from the activationsurface. Thus, with only Ni, an Ni catalyst layer is formed. However, inthis method, the dissolved Co²⁺, Mn²⁺, and so forth become oxides,thereby contaminating the surface of the alloy.

When the alloy powder is soaked in an acidic solution, the similareffect can be obtained. However, in this method, an Ni film that hardlypermeates hydrogen may be formed. When a hard Ni film is formed on thesurface of the hydrogen absorbing alloy, when it is used for a secondarybattery, the initial discharging characteristic deteriorates.

When alkali solution or an acidic solution is used for the surfacetreatment of the hydrogen absorbing alloy, the resultant solutioncontains heavy metals (Co, Al, Mn, mish metal, and so forth). Thus, atroublesome waste treatment is required.

Besides the above-described methods, Tokkaihei 3-289047 shows a methodfor improving the initial activating characteristic of the negativeelectrode of a battery. In Tokkaihei 3-289047, an electrode composed ofthe hydrogen absorbing alloy is treated with hydrogen gas so as to bethe charging state. The electrode is treated with steam that containsSO₂, CO, CO₂, and alkali mist so as to deactivate the surface of theelectrode and thereby maintain the charging state. However, in thismethod, since the surface of the electrode is deactivated, the initialdischarging capacity is not sufficiently improved.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method for producinga hydrogen absorbing alloy with an active layer that is less oxidized.Another object of the present invention is to provide a surfacetreatment method of a hydrogen absorbing alloy that is free from waste.

The present invention is a surface treatment method for a hydrogenabsorbing alloy, comprising the step of bringing steam into contact withthe hydrogen absorbing alloy in a temperature range from 200° C. to 400°C., so as to cause a surface reaction on the hydrogen absorbing alloy.Steam is contained in a gas and the gas is brought into contact with thesurface of the hydrogen absorbing alloy. The gas is an inert gas.

These and other objects, features and advantages of the presentinvention will become more apparent in light of the following detaileddescription of a best mode embodiment thereof, as illustrated in theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a saturation magnetization of a hydrogenabsorbing alloy powder produced by an activation treatment according toan embodiment of the present invention; and

FIG. 2 is a graph showing a change of a discharge capacity in an initialcycle of a nickel-metal hydride battery according to an embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the surface treatment method according to an embodiment of thepresent invention, steam is brought into contact with the surface of thehydrogen absorbing alloy in a temperature range from 200° C. to 400° C.,so as to cause a surface reaction. Examples of the hydrogen absorbingalloy are an LaNi₅ type, Ti--Fe type, and Zr alloy Laves phase type. Inparticular, an AB₅ type hydrogen absorbing alloy represented by LaNi₅ orMmNi₅ (Mm: misch metal) or an AB₂ type hydrogen absorbing alloyrepresented by TiZrVNi type Laves phase alloy such as ZrV₀.4 Ni₁.6, isused.

When steam is brought into contact with a metal, as is well known, acatalytic reaction from contact with water takes place. The water andmetal react and thereby hydrogen is generated. In addition, a metaloxide or a metal hydroxide is produced. When energy that is a trigger ofthe reaction is applied, the contact catalytic reaction of water takesplace as expressed by the following equation.

    2H.sub.2 O→2H.sub.2 +O.sub.2

In the above reaction, the temperature dependency of the chemicalreaction velocity is expressed by the following Arrhenius' equation.

    kc=A·exp(-E/RT)

where T is the absolute temperature, E is the activation energy, and Aand R are constants. Thus, when the absolute temperature T rises, thechemical reaction velocity kc increases. Consequently, the velocity ofthe catalytic reaction from contact with water increases and thereby theamount of hydrogen produced increases.

In the temperature range of 200° C. or lower, the amount of hydrogenproduced in the contact catalytic reaction is insufficient. Thus, whenan oxide is formed on the surface of the metal in the catalytic reactionfrom contact with water, the oxide is not reduced. Alternatively, theoxidizing velocity is larger than the reducing velocity. Thus, the oxide(or hydroxide) prevents the surface of the hydrogen absorbing alloy frombeing activated. (The activation of the hydrogen absorbing alloy removesan oxide that prevents hydrogen from being absorbed/released, andthereby accelerates the absorption/release of hydrogen.) In this lowtemperature range, the surface of the hydrogen absorbing alloy may bedeactivated.

On the other hand, when steam is brought into contact with the hydrogenabsorbing alloy in the temperature range of higher than 400° C., thecorrosion reaction actively progresses and thereby a thick corrosionlayer is produced. However, when the hydrogen absorbing alloy is usedfor an electrode active material of a battery, a thick corrosion layercauses the characteristics of the battery to deteriorate. In otherwords, when the degree of corrosion is large, since the amount of thehydrogen absorbing alloy that contributes to the reaction decreases, thedischarge capacity decreases. In contrast, when the degree of corrosionis large, since the surface of the hydrogen absorbing alloy is coatedwith a corrosion substance of low conductivity, the electric resistanceincreases and the discharge voltage drops. In addition, the resistancecomponent converts the charging/discharging energy into heat and therebythe discharge capacity decreases.

Moreover, in a chemical heat pump, when the amount of corrosionincreases, the amount of hydrogen absorbing alloy decreases. Thus,absorbing/releasing hydrogen decreases and thereby the amount of heatdecreases. In addition, since the surface of the hydrogen absorbingalloy is coated with a substance whose hydrogen permeability is low, theheat transmissibility decreases.

In the temperature range from 200° C. to 400° C., when steam is broughtinto contact with the surface of the hydrogen absorbing alloy, thesurface of the hydrogen absorbing alloy is activated. The followingexplanation can be assumed. In this temperature range, when thecatalytic reaction from contact with water takes place on misch metal(such as La), Mn, or Al, an oxide or a hydroxide of a metal contained inthe hydrogen absorbing alloy is produced. At this point, since asufficient amount of hydrogen is produced, the oxide or the hydroxide isreduced with the hydrogen. Consequently, the surface of the hydrogenabsorbing alloy is activated. In this reductive reaction, hydrogenreduces the Ni compound and thereby Ni metal, which is catalyticallyactive, is produced.

Thus, when steam is brought into contact with the surface of thehydrogen absorbing alloy in the temperature range from 200° C. to 400°C., the surface of the hydrogen absorbing alloy is activated and a Nicatalyst layer is produced. Thus, a good electrode active material for abattery is obtained.

The steam is contained in a gas. The gas is brought into contact withthe surface of the hydrogen absorbing alloy. This gas functions as acarrier gas that carries the steam. In addition, the gas functions as anadjustment gas that adjusts the amount of steam. By adjusting the amountof steam, the amount of the active layer of the hydrogen absorbing alloyor the Ni catalyst layer can be properly controlled. When the componentof the hydrogen absorbing alloy is changed, the thickness of the activelayer of the hydrogen absorbing alloy or the Ni catalyst layer can beflexibly controlled.

As a carrier gas for the steam, a reductive gas such as hydrogen orcarbon monoxide is preferably used. A reductive gas removes oxides fromthe surface of the hydrogen absorbing alloy. To suppress the corrosivereaction, as a carrier gas for the steam, an inert gas is preferablyused. Since an inert gas has no reaction with the hydrogen absorbingalloy, an inert gas is especially suitable as a carrier gas for thesteam, or an adjustment gas that adjusts the amount of steam.

At the present time, the surface structure of the hydrogen absorbingalloy activated by the above-described surface treatment method has notbeen sufficiently studied. However, it is considered that the structureof the hydrogen absorbing alloy activated by the surface treatmentmethod, is different from that activated with a solution (such as analkali solution). In other words, when an alkali solution or the like isused, part of a metal element composing the hydrogen absorbing alloydissolves in the solution. However, in the case of the hydrogenabsorbing alloy produced according to the present invention, no metalelement is dissolved in a solution. Thus, it is assumed that the bondingform of the elements varies between these two resultant structures.

As will be described later with experimental data, a preferabletemperature range for the steam treatment for obtaining a hydrogenabsorbing alloy for use as an active material of a battery, is from 200°C. to 300° C. However, the above-described surface treatment may beeffective under a reduced pressure environment or an increased pressureenvironment. When the pressure environment is varied, theabove-described relevant temperature range may vary. In addition, whenthe treatment apparatus varies, although the critical effect on thehydrogen absorbing alloy against the temperature does not vary, thetemperature range in which the desirable effect is obtained may vary tosome extent. When the external environment varies, the temperature rangemay change by 100° C. Thus, when a change in the external environment isalso considered, in the temperature range from 200° C. to 400° C. , thedesired effect for the surface treatment of the hydrogen absorbing alloymay be obtained.

The treatment performed in the temperature range defined in the presentinvention (namely, from 200° C. to 400° C.) may be combined with atreatment performed outside of the temperature range defined in thepresent invention. The effect of the treatment varies depending on thetemperature range. However, by properly combining the temperatureranges, the characteristics of these temperature ranges can be obtained.However, in this case, at least one of temperature ranges to be combinedshould be "the temperature range from 200° C. to 400° C.".

The hydrogen absorbing alloy that has been surface-treated (activated)in the above-described method can be effectively used as a hydrogenstoring material, heat pump, or active material of electrode.

Next, an embodiment of the present invention will be described. In theembodiment, as the hydrogen absorbing alloy, MmNi₃.6 Co₀.7 Mn₀.4 Al₀.3was mechanically crushed so that the average particle diameters thereofbecame 25 μm. In the activation treatment, a glass tube was filled withthe powder of the hydrogen absorbing alloy. While the temperature of theglass tube was kept in a predetermined range (from 150° C. to 400° C.),Ar gas passed through steam heated to 50° C. was passed through theglass tube at a flow rate of 1.8 l/min, so that the hydrogen absorbingalloy was brought into contact with the steam. After the hydrogenabsorbing alloy and steam had reacted together for approximately onehour, the powder of the hydrogen absorbing alloy was removed from theglass tube. The powder of the hydrogen absorbing alloy was dried in avacuum and thereby activation-treated powder was obtained.

To examine how the activation treatment with steam causes the surface ofthe hydrogen absorbing alloy to change, the saturation magnetization ofthe obtained hydrogen absorbing alloy was measured with avibrating-sample type magnetometer. By measuring the saturationmagnetization, the amount of ferromagnetic substances such as Co or Nicontained in the hydrogen absorbing alloy can be estimated. The state ofthe surface treatment layer was examined with the obtained values. Themeasured results are shown in FIG. 1.

As shown in FIG. 1, the saturation magnetization rapidly increases whenthe treatment temperature exceeds 200° C. When the saturationmagnetization is high, the amount of ferromagnetic substance (metal Nior metal Co) formed is large. Thus, a large Ni catalyst layer is formedand activated. When the amount of the Ni catalyst layer in the surfaceportion of the hydrogen absorbing alloy is large, if the hydrogenabsorbing alloy is used for an electrode of a secondary battery, theelectric conductivity characteristic improves. Also, when this hydrogenabsorbing alloy is used for a chemical heat pump, the heat conductivityimproves.

Thus, when the steam treatment is performed in the temperature range of200° C. or higher, contact catalyst reaction takes place between thesteam and the hydrogen absorbing alloy. Thus, a large amount of hydrogenis produced and thereby a large amount of metal Ni or Co is produced. Inother words, when a large amount of hydrogen is produced, in addition toan oxide on the surface of the hydrogen absorbing alloy, an oxide (or ahydroxide) produced by the steam reaction is also reduced by thehydrogen, and thereby metal Ni or Co is produced. Thus, when the amountof such a metal produced is large, the amount of oxide (or hydroxide) issmall.

FIG. 1 also shows the measured results of the hydrogen absorbing alloypowder on which the surface treatment has not been performed. The graphin FIG. 1 shows data of up to 300° C. At 350° C., the measured valueobtained can be seen on the extrapolated line of the curve shown in FIG.1.

Next, the characteristic in the case that the hydrogen absorbing alloyactivated by the treatment according to the embodiment of the presentinvention is used for a negative electrode of a secondary battery isshown. In other words, a negative electrode was formed with a paste ofthe hydrogen absorbing alloy powder activation-treated. With thenegative electrode and a nickel positive electrode, a nickel-hydrogensecondary battery was formed. The charging/discharging characteristic ofthe battery was examined. As samples of the hydrogen absorbing alloypowder used for the negative electrode, a non-activation-treated sample(comparison 1), an alkali-treated sample (comparison 2), and asurface-treated sample according to the embodiment, were prepared. Inthe alkali treatment, the hydrogen absorbing alloy powder was soaked ina solution of (6.8 N KOH+0.8 N LiOH) for two hours at 110° C. Thesurface-treated samples according to this embodiment were obtained bysurface-treating the hydrogen absorbing alloy powder at 150° C. for onehour, at 200° C. for one hour, at 250° C. for one hour, at 300° C. forone hour, and at 200° C. for one hour then at 280° C. for five minutes.

The negative electrode (paste negative electrode) of the battery wasformed in the following method. Each sample of the hydrogen absorbingalloy powder and a solution of methyl cellulose were mixed, and therebya paste was obtained. Each paste mixture was placed on a foamed nickelelectric collector of 30×40 mm. After the resultant substance was dried,it was compressed and a paste electrode with a thickness ofapproximately 0.6 mm was obtained. As the positive electrode, a pastenickel electrode of 45×60 mm was used.

To form a battery, two positive electrodes were prepared per negativeelectrode. The positive electrodes and the negative electrode werestacked with a separator that electrically insulated the positiveelectrodes from the negative electrode. Thus, a battery stack wasformed. As an example of the electrolytic solution, (5 N KOH+1 N LiOH)was used. In such a manner, seven types of nickel-metal hydridesecondary batteries were produced.

The charging/discharging cycles were performed under the followingconditions. The charging operation was performed at 0.2 C until thetheoretical capacity of the battery became 120%. The dischargingoperation was performed at 0.2 C until the battery output voltage became0.8 V. In these conditions, the characteristics of the initialcharging/discharging cycles of the seven types of the batteries wereexamined. "C" is the unit that represents the current of the battery."C" is used when the current is represented with the C rate. Thedefinition of the C rate is that the current by which the charge amountof the nominal capacity (Ah) of the battery is discharged in one hour is1 C. When the battery has a nominal capacity of 100 Ah, 1 C is 100 A and0.2 C is 20 A. In other words, 0.2 C is equivalent to the widely used"five hour rate capacity".

FIG. 2 shows a change in the discharge capacity of each battery in theinitial cycle. As shown in FIG. 2, the initial characteristic of thebattery using the hydrogen absorbing alloy surface-treated at 150° C. to200° C. is inferior to the initial characteristic of the battery usingthe hydrogen absorbing alloy non-activation-treated (comparison 1). Inother words, in the surface treatment of the hydrogen absorbing alloy,the contact catalytic reaction does not take place satisfactorilybetween the steam and the hydrogen absorbing alloy. Thus, a sufficientamount of hydrogen is not produced and the surface of the hydrogenabsorbing alloy is not properly activated.

The battery using the hydrogen absorbing alloy surface-treated at 250°C. has a good initial characteristic. The characteristic of this batteryis close to the characteristic of the battery using the hydrogenabsorbing alloy treated with the alkali-solution (comparison 2). Thus,according to the treatment of this embodiment, the characteristic of thealkali-treated hydrogen absorbing alloy can be obtained withoutproducing waste.

The initial activation of the battery using the hydrogen absorbing alloysurface-treated at 300° C. is high. However, the discharge capacity ofthis battery in the second or later cycles is inferior to that of thebattery using the hydrogen absorbing alloy surface-treated at 250° C.This is because in the surface treatment, the contact catalytic reactiontakes place excessively between the steam and the hydrogen absorbingalloy. Thus, the surface treatment layer becomes thick and thereby theamount of alloy that contributes to the hydrogen absorption decreases.On the other hand, a thick corrosion layer is formed.

Consequently, in the above-described surface treatment, the method forbringing steam into contact with a hydrogen absorbing alloy and causinga surface reaction in the temperature range from 200° C. to 400 C.,preferably in the temperature range from 200° C. to 300° C., ispreferably used.

Although the present invention has been shown and described with respectto a best mode embodiment thereof, it should be understood by thoseskilled in the art that the foregoing and various other changes,omissions, and additions in the form and detail thereof may be madetherein without departing from the spirit and scope of the presentinvention.

What is claimed is:
 1. A surface treatment method for a hydrogenabsorbing alloy, consisting essentially of the step of:contacting steamwith the hydrogen absorbing alloy in a temperature range from 200° C. to400° C. so as to cause a surface reaction of the hydrogen absorbingalloy to generate a sufficient amount of hydrogen and form an activesurface layer on the hydrogen absorbing alloy, wherein formation of anoxide layer is minimized.
 2. The surface treatment method as set forthin claim 1, wherein the hydrogen absorbing alloy is used for an activematerial of a battery.
 3. The surface treatment method as set forth inclaim 1, wherein the hydrogen absorbing alloy is in powder form and istreated at 200° C. for one hour.
 4. The surface treatment method as setforth in claim 1, wherein the hydrogen absorbing alloy is in powder formand is treated at 250° C. for one hour.
 5. The surface treatment methodas set forth in claim 1, wherein the hydrogen absorbing alloy is inpowder form and is treated at 300° C. for one hour.
 6. The surfacetreatment method as set forth in claim 1, wherein the hydrogen absorbingalloy is in powder form and is treated at 200° C. for one hour and thenat 280° C. for five minutes.
 7. A surface treatment method for ahydrogen absorbing alloy, consisting essentially of the stepof:contacting a gas containing steam with the hydrogen absorbing alloyin a temperature range from 200° C. to 400° C. so as to cause a surfacereaction of the hydrogen absorbing alloy to generate a sufficient amountof hydrogen and form an active surface layer on the hydrogen absorbingalloy, wherein formation of an oxide layer is minimized.
 8. The surfacetreatment method as set forth in claim 7,wherein the gas is an inertgas.
 9. The surface treatment method as set forth in claim 7,wherein thegas is a reductive gas.
 10. The surface treatment method as set forth inclaim 7, wherein the hydrogen absorbing alloy is used for an activematerial of a battery.
 11. A surface treatment method for a hydrogenabsorbing alloy, consisting essentially of the step of:contacting steamwith the hydrogen absorbing alloy in a temperature range from 200° C. to400° C. in which a metal contained in the hydrogen absorbing alloy andthe steam cause a contact catalytic reaction, in the temperature range asufficient amount of hydrogen being produced for reducing an oxideproduced in the contact catalytic reaction and forming an active surfacelayer on the hydrogen absorbing alloy , wherein formation of an oxidelayer is minimized.
 12. The surface treatment method as set forth inclaim 11,wherein in the temperature range the hydrogen absorbing alloyis not corroded by the steam.
 13. The surface treatment method as setforth in claim 11, wherein the hydrogen absorbing alloy is used for anactive material of a battery.
 14. A method of treating the surface of anhydrogen absorbing alloy which consists essentially of contacting saidhydrogen absorbing alloy with a gas mixture consisting essentially ofsteam and at least one other gas selected from the group consisting of acarrier gas, a reducing gas, and an inert gas at a temperature of from200° C. to 400° C. so as to cause a surface reaction of the hydrogenabsorbing alloy and generate a sufficient amount of hydrogen to form anactive surface layer on the hydrogen absorbing alloy, wherein formationof an oxide layer is minimized.
 15. The method as set forth in claim 14,wherein the hydrogen absorbing alloy is used for an active material of abattery.